1. Field of the Invention
This invention relates to improvements in circuitry that can be used in driving polyphase dc motors, and still more particularly to methods and apparatus for developing information concerning the crossing of a reference voltage by the back emf ( bemf) of non-selected or floating field coils for controlling a spinning rotor of a polyphase dc motor, and still more particularly to methods and apparatus for detecting the reference voltage crossings of the back emf of floating field coils for controlling coil commutation signals, even at low motor speeds.
2. Description of the Prior Art
Although the present invention pertains to polyphase dc motors, in general, it finds particular application in conjunction with three phase dc motors, particularly of the brushless, sensorless type which are used for rotating data media, as may be found in computer related applications, such as hard disk drives, CD ROM drives, floppy disks, VCRs, and the like. In such computer applications, three phase brushless, sensorless dc motors are becoming more popular, due to their reliability, low weight, and accuracy.
Usually, motors of this type can be thought of as having a stator with three field coils, generally connected in a "Y" configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. Typically, in such applications, eight pole motors are used which have four electrical cycles per revolution of the rotor. The stator coils, however, can be still be analyzed in terms of three "Y" connected coils, connected in three sets of four coils, each physically separated by 90.degree.. In operation in bipolar applications, the coils are energized in sequences in each of which a current path is established through two coils of the "Y", with the third coil left floating. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction as in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle in a three phase motor.
In the past, during the operation of a such polyphase, sensorless dc motor, it has been recognized that maintaining a known position of the rotor is an important concern. There have been various ways by which this was implemented. The most widely used way, for example, was to start the motor in a known position, then develop information related to the instantaneous or current position of the rotor. One source of such instantaneous position information was developed as a part of the commutation process, and involved identifying the floating coil, and monitoring its back emf (bemf), that is, the emf induced into the coil as it moves through the magnetic field provided by the stator.
When the voltage of the floating coil crossed zero (referred to in the art as "a zero crossing"), the position of the rotor was assumed to be known. Upon the occurrence of this event, the rotor coil commutation sequence was incremented to the next phase, and the process repeated.
However, particularly at startup of the motor, a problem exists, since the motor speed is low, and the bemf developed is very small and is difficult to detect with the sufficient accuracy to enable precision commutation of the activation voltages on the field windings of the motor. The inability to accurately determine the position of the rotor can lead to motor position inaccuracies, and even loss of synchronization. Thus, in the past, great care had to be taken to insure that the startup algorithms and running conditions were precisely controlled to avoid anything which might cause such out of synchronization condition to occur.